The US team arrived the morning after President Barack Obamaheralded “a new hope for the future of Cuba” in his State of the Union address, and said that America’s past policy of isolating the communist state has failed.

But a senior Cuban diplomat signalled that the US needed to abandon hopes of reforming the one-party regime and its centrally planned economy and cautioned that re-establishing diplomatic ties would not lead to a full relationship between the two countries.

That message, delivered as the US mission was in the air en route to Havana, appeared intended to lower expectations of reform there in the wake of December’s declaration of détente by Mr Obama and Raul Castro, the Cuban leader.

US officials say they will press human rights concerns on the trip, with Ms Jacobson scheduled to meet Cuban dissidents and religious leaders. But in another dig, Cuban officials countered that they were happy to discuss human rights and would raise their own “concerns” over US police “brutality” in Ferguson and New York, where unarmed black men were killed by white officers last year.

If the talks go well, America has been hoping to re-open its embassy building on Havana’s Malecon seafront promenade – currently home to its “interest section” – before the Summit of Americas in April in Panama, where Mr Obama and Mr Castro are expected to meet.

Cuban officials sounded less optimistic, saying that re-opening of embassies before Havana was removed from the US state terrorism sponsor list would be “contradictory”. But Cuba also needs to reach a deal as its moribund economy struggles to survive.

Despite the diplomatic jockeying, the talks mark a momentous turning point in one of the last legacies of the Cold War. Dispatching an official of Ms Jacobson’s seniority is an indication of Mr Obama’s commitment to follow through on December’s announcement.

Colleagues have praised her mix of sunny disposition and steely determination – a combination that seems certain to be needed in the coming months.

The meetings on Wednesday focused on previously planned “migration” talks covering existing travel links between the two countries. But the more challenging business begins on Thursday with the start of negotiations about re-opening embassies in the respective capitals.

Immediate US objectives for the talks include the lifting of restrictions on American diplomats’ staffing numbers and travel inside Cuba and unfettered access for Cubans to their mission. The Cuban response will help determine the pace of change.

Other potential stumbling blocks down the line include the billions of dollars in economic claims against Cuba’s government by exiles and nationalised US businesses and the status of American criminal fugitives who are now living freely in Cuba.

Some changes have already unfolded in the last month. The Cubans last week released 53 political prisoners. Three days later, the Obama administration significantly eased travel and trade rules with Cuba, including allowing American visitors to bring back $100 worth of the island’s famed cigars and rum.

The talks began with a stark visual reminder of Cuba’s history as a proxy for Cold War rivalries. Sitting in Havana harbour was a Russian spy ship that arrived on Tuesday for a port call that was a thinly veiled display of Moscow’s influence on its old communist ally.

And Ms Jacobson will not be negotiating an end to the trade embargo imposed in 1960 – the year that she was born – as only Congress can lift that.

The Republicans who control both chambers seem certain to ignore Mr Obama’s call on Tuesday night to end the sanctions put in place shortly after Fidel Castro seized power in the Cuban revolution. There is, for now at least, little prospect that Starbucks, McDonald’s or other American businesses will be setting up shop in Havana any time soon.